fix: tracking plan merged config is cached without its event type

[atzoum]

Description

DgSourceTrackingPlanConfigT.GetMergedConfig(eventType) layers the tracking plan's config for an event type over its global config, and memoised the result into the source's own DgSourceTrackingPlanConfig.MergedConfig.

Caching it there is both wrong and dangerous. Wrong, because the field has no room for the event type it was merged for, so the first caller's answer is handed to every later one:

track    -> map[allowUnplannedEvents:true propertiesRule:track-rules]
identify -> map[allowUnplannedEvents:true propertiesRule:track-rules]   <- identify's rules never appear
page     -> map[allowUnplannedEvents:true propertiesRule:track-rules]

Dangerous, because computing a merge should not write into the source config at all - and there is no need for it to: the merge is two map lookups and an lo.Assign, recomputed per event today regardless, since the memo is written once per source copy and thrown away.

It does not misfire as things stand (one event per gateway batch, a fresh source copy per job), but mergedTpConfig is what decides whether an event is dropped - rudder-transformer dispatches allowUnplannedEvents, unplannedProperties, anyOtherViolation and sendViolatedEventsTo off it, per event - so the cost of it ever misfiring is event loss rather than noise.

MergedConfig is removed with it: never on the wire in either config version, and nothing outside the getter read it. fetchEventConfig goes too - it turned an absent key into an empty map, which lo.Assign does by itself.

Security

• The code changed/added as part of this pull request won't create any security issues with how the software is being used.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.37%. Comparing base (267b973) to head (6c3b616).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7314      +/-   ##
==========================================
+ Coverage   79.34%   79.37%   +0.03%     
==========================================
  Files         609      609              
  Lines       67963    67973      +10     
==========================================
+ Hits        53922    53953      +31     
+ Misses      10876    10841      -35     
- Partials     3165     3179      +14     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[mihir20]

LGTM